418 THE EVOLUTION OF THE METAZOA 



and which is important for the correct solution of the problem 

 of the origin of the next "higher" phylum. These are two 

 phenomena which are mutually closely related. One of these 

 phenomena is connected with the way of life, above all with 

 the type of feeding and of movement, and the other pheno- 

 menon with the evolution in the direction polymerization -> 

 -> oligomerization. We find in this connection an interesting 

 and revealing survey of the development trends such as they 

 appear within the phylum Polymeria which had finally led to 

 the emergence of the next "higher" phylum. 



The starting form of the evolution of the Polymeria had been, 

 contrary to the situation which we find in the Ameria, a bilate- 

 ral richly segmented and therefore elongated eumetazoan which 

 had moved over the surface of the sea bottom by means of a 

 sinuous type of locomotion and which fed as a predatory 

 animal. The polymerization took place in a part of their 

 subline of evolution which developed progressively; its con- 

 sequence are species which can reach a length of several 

 metres, which consist of hundreds of segments, and which 

 are either equipped with parapodia (Polychaeta errantia), or not 

 at all (Oligochaeta). In the shorter side lines of evolution, 

 however, the animals have actually preserved their free mobili- 

 ty while at the same time the size ot their body was considera- 

 bly decreased and the number of their segments reduced. They 

 had thus been subjected to oligomerization. This development 

 had taken place both in the Polychaeta and in the Oligochaeta. 

 There have also been some instances of neoteny. 



Another part of the freely moving Polymeria also developed 

 progressively ; they changed their type of movement by de- 

 veloping lateral excrescences which evolved into active 

 extremities. Here the number of segments had been reduced 

 (oligomerization) in several sublines of evolution, even in 

 those whose representatives had begun to live on land; this 

 reduction was usually combined with a differentiation of vari- 

 ous parts of segments which led to a regionalization of the 

 body, and above all to the cephalization of the foremost 



